This invention relates to the refining of petroleum and more particularly to the removal of finely divided solid particles from liquid hydrocarbon fractions.
In the refining of petroleum, an initial step is to distill the petroleum to separate the oil into a number of fractions by virtue of the difference in their boiling points. Some of the fractions from the distillations are further processed by passing them through fixed beds of catalysts under conditions of temperature and pressure, and frequently in the presence of hydrogen, to convert the petroleum fractions to products of higher quality. For example, virgin naphthas separated from crude oil by distillation may be passed through a pretreater containing a fixed bed of catalyst to remove sulfur and nitrogen compounds and then through a reformer. Kerosene and residual oil from the distillation may be subjected to hydrodesulfurization by passing through a fixed bed of catalyst to produce jet fuels or fuel oils of higher quality. Gas oil from the distillation may be passed through a catalytic cracking unit in which part of the gas oil is converted to gasoline and more volatile hydrocarbon fractions and a light gas oil. The catalytically cracked light gas oil may then be hydrocracked by passing it through a fixed bed of hydrocracking catalyst at elevated temperatures and pressures in the presence of hydrogen.
Even though the concentration of solid particles in the liquid hydrocarbon fractions is low, during the long runs through some fixed beds of catalyst the solids may be deposited on the catalyst and can plug beds and necessitate shutting down the process for replacement of at least a part of the catalyst before the catalyst is spent. The loss of production and the direct labor and catalyst replacement costs make the more frequent catalyst replacement very costly. The solid particles may in some instances, and particularly in reduced crudes, be solid particles that were in the crude oil charged to the distillation unit; however, a large part of the solid particles in distillate products from the atmospheric distillation are electrically conductive materials such as iron oxide or iron sulfide particles picked up from the processing vessels.
The size of the suspended solid particles is often extremely small. In some hydrocarbon fractions, for example the charge stock to a hydrocracker, 98 percent of the particles have a diameter less than 5 microns and a major part of the particles have a diameter less than one micron. Such particles do not settle from the hydrocarbon liquids. Filtration of the liquid by passing liquid through a permeable medium is not effective. If the openings in the filter medium are small enough to trap the solid particles, the filter medium quickly becomes plugged. Moreover, most of the liquid hydrocarbon streams in a refinery are hot, and the conventional filter media, such as paper or urethane foam, are not capable of withstanding the high temperature.